1. Field of the Invention
The present invention relates to photolithographic systems used for fabricating integrated circuit devices, and more particularly, to photolithographic systems that compensate for lens errors.
2. Description of the Related Art
An insulated-gate field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source, and drain are located in a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, that sets up a transverse electric field in order to modulate the longitudinal conductance of the channel.
Polysilicon (also called polycrystalline silicon, poly-Si or poly) thin films have many important uses in IGFET technology. One of the key innovations is the use of heavily doped polysilicon in place of aluminum as the gate. Since polysilicon has the same high melting point as a silicon substrate, typically a blanket polysilicon layer is deposited prior to source and drain formation, and the polysilicon is anisotropically etched to provide the gate. Thereafter, the gate provides an implant mask during the implantation of source and drain regions, and the implanted dopants are driven-in and activated using a high-temperature anneal that would otherwise melt the aluminum. This self-aligning procedure tends to improve packing density and reduce parasitic overlap capacitances between the gate and the source and drain. As such, the gate length (or xe2x80x9ccritical dimensionxe2x80x9d) has a major influence on the channel length.
The performance of an integrated circuit depends not only on the value of the channel lengths, but also upon the uniformity of the channel lengths. In an integrated circuit having some devices with relatively longer channel lengths and other devices with relatively shorter channel lengths, the devices with shorter channel lengths have a higher drain current than the devices with the longer channel lengths. The difference in drain currents can cause problems. For instance, devices with too large a drain current may have a high lateral electric field that causes significant hot carrier effects despite the presence of a lightly doped drain (LDD), whereas devices with too small a drain current may have unacceptably slow switching speeds. Therefore, accurate gate lengths can be extremely important to achieving the required device performance and reliability.
Photolithography is frequently used to create patterns that define where a polysilicon layer is etched to form the gates. Typically, the wafer is cleaned and prebaked to drive off moisture and promote adhesion. An adhesion promoter is deposited on the wafer and a few milliliters of positive photoresist are deposited onto the spinning wafer to provide a uniform layer. The photoresist is soft baked to drive off excess solvents. The photoresist is irradiated with an image pattern that renders selected portions of the photoresist soluble. A developer removes the soluble portions of the photoresist and an optional de-scum removes very small quantities of photoresist in unwanted areas. The photoresist is hard baked to remove residual solvents and improve adhesion and etch resistance. The etch is applied using the photoresist as an etch mask, and the photoresist is stripped. Therefore, the photoresist has the primary functions of replicating the image pattern and protecting the underlying polysilicon when etching occurs.
Photolithographic systems typically use a light source and a lens in conjunction with a mask or reticle to selectively irradiate the photoresist. The light source projects light through the mask or reticle to the lens, and the lens focuses an image of the mask or reticle onto the wafer. A mask transfers a pattern onto the entire wafer (or another mask) in a single exposure step, whereas a reticle transfers a pattern onto only a portion of the wafer. Step and repeat systems transfer multiple images of the reticle pattern over the entire wafer using multiple exposures. The reticle pattern is typically 2xc3x97 to 10xc3x97 the size of the image on the wafer, due to reduction by the lens. However, non-reduction (1xc3x97) steppers offer a larger field, thereby allowing more than one pattern to be printed at each exposure.
Photolithographic systems often use a mercury-vapor lamp as the illumination source. In mercury-vapor lamps, a discharge arc of high-pressure mercury vapor emits a characteristic spectrum that contains several sharp lines in the ultraviolet regionxe2x80x94the I-line (365 nm), the H-line (405 nm) and the G-line (436 nm). Photolithographic systems are designed, for instance, to operate using the G-line, the I-line, a combination of the lines, or at deep ultraviolet light (240 nm). To obtain the proper projection, high power mercury-vapor lamps are used that draw 200 to 1000 watts and provide ultraviolet intensity on the order of 100 milliwatts/cm2. In some systems, air jets cool the lamp, and the heated air is removed by an exhaust fan.
The reticle typically includes a chrome pattern on a quartz plate. The chrome pattern has sufficient thickness to completely block ultraviolet light, whereas the quartz has a high transmission of ultraviolet light. Although quartz tends to be expensive, it has become more affordable with the development of high quality synthetic quartz material.
Lens errors in step and repeat systems are highly undesirable since they disrupt the pattern transfer from the reticle to the photoresist, which in turn introduces flaws into the integrated circuit manufacturing process. Lens errors include a variety of optical aberrations, such as astigmatism and distortion. Astigmatism arises when the lens curvature is irregular. Distortion arises when the lens magnification varies with radial distance from the lens center. For instance, with positive or pincushion distortion, each image point is displaced radially outward from the center and the most distant image points are displaced outward the most. With negative or barrel distortion, each image point is displaced radially inward toward the center and the most distant image points are displaced inward the most.
Replacing the lens in a step and repeat system is considered impractical since the lens is a large, heavy, integral part of the system, and is usually extremely expensive. Furthermore, it is unlikely that a substitute lens will render subsequent corrections unnecessary. Accordingly, the lens error can be measured so that corrections or compensations can be made.
A conventional technique for evaluating lens errors includes performing a photoresist exposure and development using specially designed mask patterns to be used for evaluation purposes. After such an imaging process, the wafer is either subjected to an optical inspection or is further processed to form electrically measurable patterns. The use of photosensitive detectors fabricated on silicon to monitor optical systems is also known in the art. For instance, U.S. Pat. No. 4,585,342 discloses a silicon wafer with light sensitive detectors arranged in a matrix, an x-y stage for positioning the wafer so that each one of the detectors is separately disposed in sequence in the same location in the field of projected light, and a computer for recording the output signals of the detectors in order to calibrate the detectors prior to evaluating the performance of an optical lithographic system.
After the lens error is measured, some form of corrective measure is typically employed. For instance, U.S. Pat. No. 5,308,991 describes predistorted reticles that incorporate compensating corrections for known lens distortions. Lens distortion data is obtained which represents the feature displacement on a wafer as a function of the field position of the lens. The lens distortion data is used to calculate x and y dimensional correction terms. The inverted correction terms are multiplied by a stage controller""s compensation value to correctly position the reticle. In this manner, the reticle is positioned to compensate for the lens error. A drawback to this approach, however, is that a highly accurate reticle positioning apparatus is required. Furthermore, although feature location can be adjusted, it is difficult to adjust the feature size.
The stepper focus setting corresponds to the adjustable distance between the wafer surface and the reticle/lens. Unfortunately, the lens error creates focusing variations, and it becomes difficult or impossible to properly focus the entire exposure field. xe2x80x9cBest focusxe2x80x9d is the focus setting that provides the best resolution and linewidth control. Best focus usually optimizes the focus near the center of the exposure field, but in doing so, often creates a substantial focusing error near the periphery of the exposure field. The focusing error tends to expand the image pattern, which can decrease the photoresist linewidth, leading to decreased gate lengths and corresponding variations in channel lengths and drain currents.
While optical photolithography continues to be the dominant technology because it is well established and is capable of implementing sub-micron resolution at least as low as 0.35 microns using current equipment, as feature sizes approach 0.5 microns and below, and these features extend across wafer areas of a square inch and more, extensive efforts are being directed at developing alternative technologies. Electron-beam, ion-beam, and x-ray technologies have demonstrated patterning capabilities that extend beyond the limits of optical systems. Electron-beams and ion-beams can also directly write image patterns onto the photoresist without the use of a mask or reticle, for instance by using a controlled stage to position the wafer beneath the tool. However, these alternative approaches have drawbacks. For instance, electron-beam lithography has low throughput, x-ray lithography has difficulties with fabricating suitable masks, and ion-beam lithography has low throughput and difficulties with obtaining reliable ion sources.
Thus, workers in the art recognize that there are obvious incentives for trying to push the currently dominant technology (optical photolithography) into the fine-line region. Such an effort, if successful, has the potential for retrofitting or modifying expensive equipment to give it significantly better patterning capabilities.
Accordingly, a need exists for improvements in photolithography that facilitate forming fine-line patterns, that are well-suited for optical photolithographic systems that pattern integrated circuit devices, and that compensate for lens errors. One desirable feature of an improved photolithography would be the reduction of the effects of lens errors in photolithographic systems such as step and repeat systems during the fabrication of integrated circuit devices. Another desirable feature of an improved photolithography would be a convenient technique for upgrading existing photolithographic systems.
The foregoing and other features are accomplished, according to the present invention, by using a specially designed light filter in a photolithographic system. The light filter varies the light intensity according to measured dimensional data that characterizes the lens error.
In accordance with one aspect of the invention, a method of compensating for a lens error of a lens in a photolithographic system includes characterizing the lens error in terms of measured dimensional data as a function of x and y coordinates on an exposure field associated with the lens, providing a light filter designed to vary light intensity in accordance with the measured dimensional data, and projecting light through the light filter, a reticle and the lens to form an image pattern on a positive photoresist layer during the fabrication of an integrated circuit device. The light filter compensates for the lens error by reducing the light intensity of the image pattern as the lens error increases.
In this manner, when the lens error causes focusing variations that result in enlarged portions of the image pattern, the light filter reduces the light intensity transmitted to the enlarged portions of the image pattern. This, in turn, reduces the rate in which regions of the photoresist layer beneath the enlarged portions of the image pattern are rendered soluble to a subsequent developer. As a result, after the photoresist layer is developed, linewidth variations that would otherwise result from the lens error are reduced due to the light filter.
The invention is particularly well-suited for patterning a photoresist layer that defines polysilicon gates for an integrated circuit device. The light filter compensates for the lens error, thereby reducing linewidth variations in the photoresist, reducing variations in the gate lengths, and ultimately reducing variations in channel lengths and drive currents. Of course, the invention is well-suited for patterning photoresist layers that define other circuit elements, particularly where highly accurate pattern transfer is necessary.
Preferably, the light filter includes a light-absorbing film on a light-transmitting base, and the thickness of the light-absorbing film varies in accordance with the measured dimensional data. That is, variations in the thickness of the light-absorbing film are responsible for variations in the light intensity of the image pattern. It is also preferred that the thickness of the light-absorbing film is determined by deriving light intensity data from the measured dimensional data using a curve obtained by previous measurements of patterns fabricated by the photolithographic system, and deriving thickness data from the light intensity data using Lambert""s law of absorption. As exemplary materials, the light-absorbing material is a semi-transparent layer such as calcium fluoride, and the light-transmitting base is a quartz plate.
In accordance with another aspect of the invention, obtaining the measured dimensional data includes projecting light through a test pattern and the lens to provide a first image pattern on a first photoresist layer, developing the first photoresist layer to form openings therethrough that correspond to the first image pattern such that the first photoresist layer defines a transfer pattern, and measuring the transfer pattern. The first photoresist layer can be measured directly. Alternatively, the method may include etching a test material through the openings in the first photoresist layer using the first photoresist layer as an etch mask to remove selected portions of the test material, thereby forming test segments from unetched portions of the test material, stripping the first photoresist layer, and measuring the test segments.
Advantageously, the light filter can be installed in conventional photolithographic systems, thereby providing a relatively convenient and inexpensive technique for improving the patterning capability of existing photolithographic systems.
These and other features and advantages of the invention will be further described and more readily apparent from a review of the detailed description of the preferred embodiments which follow.